ztest: setup a default app memory domain

If CONFIG_APP_SHARED_MEM is enabled, ztest will set up a
default memory domain with a single partition on startup.
Test case globals may be added to the partition via
ZTEST_BMEM/ZTEST_DMEM macros, add their own partitions,
or leave the domain and join another one.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>